Tool Kit (Tk) is a companion program to Tool Command Language (Tcl) for creating graphical user interfaces. Together with Tcl, Tk is a rapid program development tool from Scriptics. Both can be ordered from the Scriptics Web site.
Tk provides a collection of standard buttons, menus, listboxes, and scrollbars and two special features, the text widget and the canvas widget, that the Tcl programmer can use to speed up program development.
In a more general sense, tool kit is synonymous with software development kit (SDK).